Robert Hammond and Joshua David co-founded Friends of the High Line, the organization responsible for converting an old elevated West Side railway into one of the greatest, most innovative public parks in the world. The two friends rescued the rail line from destruction, and we will always be grateful that they happened to take up the cause. To take a stroll along the 1.45-mile stretch of raised sidewalk that makes up the High Line, dotted with plants and stones and unusually shaped benches, is to experience the city in an entirely novel way. The High Line, which runs through the heart of Chelsea, has spurred a good deal of innovation and commerce in the city just through its presence. The third and final phase of the walkway, which starts at 30th Street and 10th Avenue, is slated to be completed next year; the end of that portion will wrap around an ambitious development of skyscrapers in the Hudson Yards. Jeff Koons has expressed interest in dangling a full-size replica of a steam locomotive on a crane over the High Line for one of his thought-provoking public art installations. Critics have said the High Line feels too much like a museum, that it is too sanitized, that it has accelerated the process of gentrification in New York. And they may be right. At the very least, though, the park (which is not entirely unprecedented—the Promenade plantée, in Paris, came before it) provides pedestrians with a unique vantage point on the city, one that is hard to access anywhere else. And that’s all thanks to Mr. Hammond and Mr. David, who serendipitously met at a community board meeting in 1999. The High Line makes you rethink New York City—what it’s composed of, how it works and how it could be different.